1. Field of the Invention
The present invention relates to real time electrooptical imaging for the purposes of object detection and/or tracking systems and navagation. More particularly, the invention relates to an automatic light control system to regulate entry of light and television camera tube voltages, for example, for controlling the iris of a lens assembly in conjunction with nonlinearly and stably controlling the accelerating voltage of an SEC vidicon and protecting it against damage under a wide range of ambient relatively steady state and transient light conditions.
2. Description of the Prior Art
An SEC vidicon target can suffer a permanent reduction in gain as a result of excessive energy falling on the target. This is reported in "The Effect of Intense Exposure on SEC Targets," A. H. Boerio, Westinghouse Electronic Tube Division, Elmira, New York, Report No. ET-5, dated Apr. 13, 1965. Recent SEC vidicon tube data include more specific operational limitations. Though optimum operation is far below these exposure levels, a reliable means must be provided for sensing possible transient exposures at damaging levels. The exceptionally large dynamic range of the camera system makes it imperative to protect the vidicon against possible over-exposures of the system from large scene light level changes, maintenance procedures, (inadvertent operation with critical circuit boards removed, etc.), and component failures. The light level dynamic range over which the camera system sensitivity must be controlled is determined as follows: The maximum apparent scene brightness that the system can tolerate under full gain conditions is determined by the saturation and permanent "burn-in" light level of the SEC vidicon. The expression for the scene brightness for this condition is given by: EQU B.sub.HIGH GAIN = [4(f/no).sup.2 E.sub.K ]/.tau.(G.sub.max), ft.- lamberts
where
f/no = aperture ratio of the lens = 1.3 for example PA1 .tau. = total system transmittance = 0.7 for example PA1 E.sub.k = saturation faceplate illumination level of SEC vidicon PA1 = 0.53 .times. 10.sup.-2 ft. candles for example PA1 G.sub.max = maximum intensifier luminance gain = 400(0.4) PA1 = 160 for example
which gives EQU B.sub.HI GAIN = 0.32 .times. 10.sup.-3 ft.-lamberts
The specified maximum scene brightness level over which the camera system must operate may be B.sub.LO GAIN = 1 .times. 10.sup.4 ft.-lamberts, for example. Thus, the required dynamic range for ALC is EQU (B.sub.LO GAIN /B.sub.HI GAIN ) = (1 .times. 10.sup.4)/(0.32 .times. 10.sup.-3) = 3.1 .times. 10.sup.-7
automatic rapid control of the system sensitivity over a wide dynamic light-level range while providing adequate camera tube protection requires gain and attenuation control in various system elements. The nature of these controls is determined by the light level dynamic range and the optimum operating points of the camera tube, e.g., the SEC vidicon tube.
Prior art methods of adjusting and protecting television camera tubes such as vidicon tubes from the effects of excessive light input have been known. Such systems include automatic gain control loops or light entry control means. Prior art systems do not provide for control of both the iris and the vidicon tube while providing a non-linear control of the vidicon. Further, such systems are not stably operative over a wide dynamic range of ambient light conditions including starlight to dusk or starlight to sunlight. Prior art systems do not provide the stable, high speed and safe operation provided by the system of this invention.